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Positive to Negative

  • September 14, 1999
  • By Joe Burns
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I can't drive Tip 55...

There's one more thing to worry about in this here calculator script and that is the button that changes numbers from negative to positive. Remember in the first Tip in this series we saw two text boxes and one was hidden? Now that all comes into play.

Let's take a final look at it all.

See It In Action


Here's the Code

(It's big. Get it all.)

Three codes will come into play here. The first is the button that triggers the function that changes a number from positive to negative or back again. It has the value of +|- on the front and the code looks like this:

 

<INPUT TYPE=BUTTON VALUE=" +|- " NAME="sign" onClick="change()">


The button itself isn't all that hard to understand. All it does is enact a function called change(). That function looks like this:

 
	 
	 function change()
{
var temp = document.calculator.text.value;

if (temp.substring(0,1) == "-") 
{document.calculator.list.value = "";
document.calculator.text.value = 0 - document.calculator.text.value * 1}

if (temp.substring(0,1) != "-") 
{document.calculator.list.value ="";
document.calculator.text.value = "-" + temp.substring(0,temp.length)}
}
	 
	 
Now, remember, that the display above is a little cramped. Make sure the function is in the format shown in the script code.

Let's take a look at what happens. When the button is clicked, the value that appears in the display (document.calculator.text.value) is assigned the variable name "temp".

Next, the script tests to see if the value is already negative. An If statement asks if the text.substring's first character is equal to the minus sign. Remember that any time you set a substring, you have to set up a comparison where the larger number wins. In this case the two numbers are 0 and 1. The one is bigger, thus it wins and the first character in the substring is checked.

Notice the two equal signs. That means "is equal to."

If the first character is equal to the minus sign, the hidden text box comes into play. Here's that code:

 	 
<INPUT NAME="list" TYPE=HIDDEN>
<INPUT TYPE=TEXT NAME="text" VALUE="">	 
	 
I have listed the hidden box and then the display text box. Both of them come into play here. The hidden text box is given the name "list." Now you can get at the box using the hierarchy statement document.calculator.list.value.

So, back to If The First Character Is The Minus Sign. If it is, then the hidden text box is given a value of nothing. By doing that it brings it into play. That hidden box is then set to 0, the value in the display is taken away, and the result is timed by 1. That sets the same number to a positive value.

Moving along...

If the first character is not a minus sign, the second If statement comes into play. This statement asks if the first character is NOT equal to a minus sign. (!= is the operator for "is not equal to").

If this If statement is true, then, again, the hidden box is given a value of nothing to bring it into play. The value of the hidden box is then set to a minus sign plus the full length of the "temp" variable substring.

I know that seems a bit much, but by adding the value through a substring, the value remains the same. Rather than the entire thing being seen as one entity, which it is not, it remains a string of characters, which it is.

And that wraps up the calculator. It was quite involved and sometimes a little rough to get hold of, but now that you're through it, I think you'll agree it was a doable project.

Next time we'll get into another great script! It allows you guitarists to choose a chord and the script then displays it. The reason I chose it is not for its function as much as because the author actually used JavaScript to build his HTML page. We'll get into it next week.

Next Time: Guitar Chords in JavaScript






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